The development of a system for laser hydrography that has sufficient accuracy to meet the standard requirements of the international hydrographic community must incorporate an allowance for the variation (mostly elongation) of the effective path length of the laser beam as it passes through the sea. The variation arises as light photons interact with particles and other scattering centers within the body of the ocean, and are either absorbed or scattered in a possibly different direction to the direction of incidence.
The effective net path variation, as viewed by an airborne receiver, will depend on sea depth, the scan angle at which the laser pulse is fired relative to the local vertical, the concentration and nature of hydrosols within the ocean causing scattering of the laser beam, as well as a number of other factors.
This general field was described in the specification of a Patent Application field under the Patent Cooperation Treaty, International Publication Number WO 82/01075 which related to Ocean Depth sounding from the air by laser beam which used a red and green laser beam and received reflected signals back by means of a pair of telescopes, one receiving the infrared signal and the other the green signal. The work which resulted in the Application was undertaken by what is known as the WRELADS group of the Defense Department of the Commonwealth of Australia.
Analysis of the large bank of data collected by the WRELADS laser hydrographic system shows that consideration of sea depth and scan angle are not sufficient to meet accuracy requirements, and that account must be taken of temporal as well as geographic variations in sea turbidity.
Sea turbidity can be chracterized by what is commonly referred to as the inherent optical properties. These are the absorption coefficient "a", the scattering coefficient "b", and the volume scattering function (.beta.(.theta.), where .theta. is the angle of scattered photon to its direction of incidence on a scattering center). The volume scattering function is on many occasions approximated by the two components ##EQU1## b.sub.f and b.sub.b are referred to as the forward scattering and the backscatter coefficients respectively, and the forward scatter is generally very much greater than the backscatter.
This invention describes a method for the real time estimation of the scatter "b" by airborne laser hydrographic systems to an accuracy sufficient for incorporation within a predictive model of the process of photon path variation by scattering.
It is noted that the process of backscatter within the sea bulk will lead to the detection by an airborne system of what is commonly referred to as a backscatter envelope. Studies from both a theoretical basis and an experimental basis as carried out by WRELADS show that the shape of this envelope under normal conditions of a uniform mixture of hydrosols within the vertical column of seawater traversed by the laser beam is of an initial high point followed by an exponential decay. Under conditions of constant system gain and laser power, the peak height of the envelope will be proportional to the backscatter coefficient. The exponential decay is characterized by the decay constant 2k, which is referred to as the attenuation coefficient.
Theoretical studies, using Monte Carlo techniques and assumed volume scattering functions, have shown that "k" is a function of both "a" and "b", but under normal conditions in hydrographic laser systems such as WRELADS, the field of view of the receiver is sufficiently large for k=a to be a good approximation. These theoretical studies also show, however, that in the limit of a very small field of view, then "k" approaches c=a+b. "c" is sometimes referred to as the total (or beam) attenuation coefficient, since it represents the decay constant for energy in the laser beam associated with photons that have been neither scattered nor absorbed as they pass downwards through the sea.
Studies to date using WRELADS data have concentrated on using measurements of "k", and hence "a", together with a parameter proportional to the backscatter envelope amplitude, and hence b.sub.b to make inferences about changes in "b", and hence in the light path variation. However, while these studies have shown that these inherent optical properties may be functionally linked within a limited time and space, the link is insufficient to make the general inferences that would be required in an operational system of air borne laser hydrography.